(1) Field of the Invention
The present invention relates to a capacitance type accelerometer and an air bag system utilizing the same.
(2) Description of the Prior Art
A capacitance type accelerometer is fundamentally composed of a movable electrode (mass portion) supported by an elastic supporting member (for instance, a cantilever, a diaphragm, etc.), and a fixed electrode facing the movable electrode with a small gap thereinbetween. When an inertial force caused by acceleration is applied to the movable electrode, the movable electrode is displaced to a position where the inertial force balances with restitution force of the elastic supporting member (an elastic body), a value of capacitance between the movable electrode and the fixed electrode is changed according to the acceleration, and the acceleration can be determined from the value of the capacitance.
The above described kind of accelerometer is noteworthy as capable of being miniaturized and having high precision.
A capacitance type accelerometer in the prior art is disclosed, for example, in JP-A-60-244864 (1985).
In considering dynamic operation of a capacitance type accelerometer, when a movable electrode is displaced in a direction thereby reducing the gap between the movable electrode and fixed electrode, viscous gas existing in the narrow gap between the electrodes is pushed out from the gap, and a large resisting force in a direction opposite to the displacing direction is caused on the movable electrode by the squeeze film effect of viscous gas. On the other hand, when the movable electrode is displaced in a direction to thereby widen the gap therein between, a large resisting force in a direction to hinder the dislocation is caused by XYZ. The above described resisting forces are so-called air damping, and result in a lower response of the sensor.